The present invention generally relates to an air-fuel ratio control system for an automobile internal combustion engine and, more particularly, to an air-fuel ratio control system incorporating an altitude compensator for optimally controlling the air-fuel mixing ratio of a combustible air-fuel mixture while the engine is operated at a high altitude.
Those skilled in the art are familiar with the fact that, in an automobile exhaust system including a three-way catalytic converter effective to minimize not only CO and HC components but also NOx component of the exhaust gases, the three-way catalytic converter can work at its maximum conversion efficiency if the air-fuel mixing ratio of the combustible mixture to be, or being supplied to the engine is controlled so as to be a stoichiometric value. A prior art air-fuel ratio control system embodying this concept comprises a composition sensor effective to detect the concentration of a component, for example, oxygen, contained in the engine exhaust gases and to generate an output signal representative of a function of the air-fuel mixing ratio of the combustible mixture burned in the engine, an air-fuel ratio control including a comparator, a proportionating circuit, an integrator, an adder and a duty radio control circuit which is capable of generating an output signal, the magnitude of which is in dependence on the output signal from the composition sensor, that is, the concentration of the exhaust gas component, and an actuator provided in an automobile carburetor and operable in response to the output signal from the ratio control circuit to adjust the air-fuel mixing ratio by adjusting the amount of bleed air to be admixed with fuel before the fuel is mixed with a primary air in the carburetor.
The U.S. Pat. No. 4,111,170, patented on Sept. 5, 1978, discloses a technique of adjusting the air-fuel mixing ratio by preventing air stepwisely or continuously drawn into a carburetor main fuel passage from being undesirably increased with increases in engine load. This patent discloses the use of a controller capable of comparing the output signal from an exhaust composition sensor, which is representative of the sensed air-fuel mixing ratio of the combustible mixture burned in the engine, with a set or desired air-fuel mixing ratio and generating first and second command output signals when the sensed air-fuel mixing ratio is lower and higher than the set or desired air-fuel mixing ratio, respectively, and a pulse generator operable in response to the first and second command output signals for generating first and second pulse signals, respectively. The first pulse signals are fed to both of first and second electromagnetic flow control valves effective to supply bleed air into the carburetor main and low speed fuel passages, respectively, and are utilized to increase the ratio of the open time of the flow control valves to their closed time to increase the flow of bleed air into the main and low speed fuel passages. On the other hand, the second pulse signals are also fed to the flow control valves, but are utilized to reduce the ratio of the open time of the flow control valves to their closed time to reduce the flow of the bleed air into the main and low speed fuel passages.
The automobile air-fuel ratio control system incorporating with an altitude compensator for optimally controlling the air-fuel ratio while the engine is operated at a high altitude where the density of air and, therefore, the atmospheric pressure is lower than at a level land is disclosed, for example, in the U.S. Pat. No. 4,086,890, patented on May 2, 1978.
As is well known to those skilled in the art, when the automobile engine is operated under a high load, a relatively enriched combustible mixture has to be supplied to the engine in order to maintain an optimum power output constantly during such high load operating condition. However, when the high load operating condition occurs while a vehicle is driven at a high altitude, the relatively enriched combustible mixture tends to be further enriched, that is, the combustible mixture tends to be excessively enriched, because of the reduced atmospheric pressure. Once the combustible mixture is excessively enriched as hereinabove described, not only does waste of automobile fuel occur, but also a relatively large amount of noxious components of the exhaust gases are emitted to the atmosphere.
Moreover, without the altitude compensator, if the driver of a vehicle running at a high altitude desires to appreciate a drivability similar to that available when the vehicle is driven at a level land, the air-fuel ratio control system is operated to enrich the combustible mixture in view of the fact that the atmospheric pressure at the high altitude is lower than at the level land and the charge efficiency of the engine is therefore lowered. Accordingly, the air-fuel ratio control system tends to operate even when the engine of the vehicle running at the high altitude is operated under such an engine operating condition in which it ought not to operate during the running of the vehicle at the level land. Thus the frequency at which the air-fuel ratio control system is operated to enrich the combustible mixture increase with the increased amount of fuel required and also with the increased emission of the pollutants of the exhaust gases to the atmosphere.
The altitude compensator disclosed in the second-mentioned U.S. patent is operable to avoid any possible excessive enrichment of the combustible mixture and is not operatively associated with the air-fuel ratio control system in the sense that they are actuated by the utilization of separate control signals which are electrical and mechanical ones. In other words, the altitude compensator disclosed therein itself operates independently of the actuator for controlling the supply of bleed air, and an electric control signal generated in dependence on the sensed concentration of oxygen contained in the exhaust gases is not utilized to control the operation of the altitude compensator.